Packing material having expanded graphite dispersed throughout

ABSTRACT

A minimum leakage packing system is provided for sealing pump shafts and the like. The packing is manufactured from twisted, exfoliated extruded, pultruded or slit graphite material that is braided, twisted, laid up or otherwise combined to form mechanical packing. A lubricant and/or sealant may be applied to the finished packing. Graphite foil may be applied to the packing for enhanced properties. The packing system preferably consists of a precision wedge set and at least three rings including compressible graphite material, the rings compressed to different percentages of their original heights. The outside rings will have the most compression and the inner ring or rings of the multi-ring assembly the least. All should be compressed less than the maximum amount to permit high conformance to the surface being sealed. The system may have an optional gasket washer following the last ring of compressed graphite material.

BACKGROUND OF THE INVENTION

The invention relates generally to a packing system and moreparticularly to material for sealing and preventing leakage around pumpshafts, rods and the like.

Many industrial processes use a water suspension system to movematerials or manufacture final products. Examples of industries wherethis is common include paper manufacturing, domestic and industrialwaste disposal and mining. Each of these industries employ processeswhere suspensions are moved with pumps. Other industrial processesemploy pumps to move large quantities of liquids for various otherpurposes.

A conventional pump includes a motor and some sort of impeller, such asa blade or piston. The motor is typically outside of the flow of fluidand the impeller is typically exposed to the flow of fluid. Energy istypically provided from the motor to the impeller through a shaft orrod. It is thus necessary to seal the fluid being pumped from theopening for the shaft, while permitting the shaft to spin or reciprocateat high speeds for long durations of time.

One method of sealing shafts is to provide a mechanical seal in the formof a high precision machined rotating disk with a matching sealing faceprecisely fitted to the rotating shaft. However, such a construction isexpensive to manufacture, frustrating to maintain and difficult torepair. Thus, the use of such mechanical seals is limited.

A more common approach is to employ a stuffing box filled with aconventional braided packing material. Such material surrounds the shaftand permits the shaft to rotate therein while substantially preventingliquids or gases from leaking out of the pump housing. Conventionalstuffing boxes for many pumps are designed to permit a small amount offluid to leak through the stuffing box and out of the pump. The fluidleakage is permitted to increase the lubrication properties of thestuffing material and provide for heat transfer. When dangerous orinconvenient fluids or suspensions are being pumped, it is also commonto provide the stuffing box with a water inlet, to permit water flowthrough the stuffing box, isolating the problem fluid, while allowingleakage of water to provide lubrication and cooling of the packing.

In each of these conventional operations involving the controlledleakage of liquid through the packing material, it is not uncommon for1-3 gallons per day or more of liquid to be consumed and/or leak out ofthe stuffing box. For a facility having 1,000 pumps or more, it is clearthat the total daily amount of liquids leaking from pumps can be quitehigh.

In recent years, the cost of water in most industrial processes hasrisen significantly and the cost of cleaning up the wastewater ofindustrial processes has risen even more. During the manufacture ofpaper, the treatment of domestic and industrial waste and mining, muchof the water leaking through normal braided packing installations iscontaminated and requires significant cleanup procedures before it canbe discharged back to natural sources, such as rivers, lakes andstreams. Vapors and other undesirable emissions are also permitted toescape from conventional pumps and valves.

Accordingly, it is desirable to provide a packing material that canovercome drawbacks of the prior art.

SUMMARY OF THE INVENTION

Generally speaking, in accordance with the invention, a substantiallyleak free waterless packing material and system are provided for sealingpump shafts and the like. The packing is manufactured from twisted,exfoliated, extruded, pultruded or slit graphite material that isbraided, twisted, laid up or otherwise combined to form mechanicalpacking. A lubricant and/or sealant may be applied to the finishedpacking. Graphite foil may be applied to the packing for enhancedproperties. The packing preferably is formed with at least three ringsincluding compressible graphite packing, the rings compressed todifferent percentages of their original heights. The outside rings willhave the most compression and the inner ring or rings of the multi-ringassembly the least. All should be compressed less than the maximumamount possible to permit high conformance to the surface being sealed.

Accordingly, it is an object of the invention to provide improvedpacking material.

Another object of the invention is to reduce or eliminate the amount ofleakage of liquids and vapors from pump seals.

Another abject of the invention is to eliminate the need for a flush orbarter fluid, injected into the middle of the stuffing box, resulting inthe dilution of the process fluid and additional leakage from the pumpor rotating shaft.

A further object of the invention is to provide an improved method ofmanufacturing packing material.

Still another object of the invention is to provide an improved packingproduct to significantly limit or eliminate leakage through seals.

Yet another object of the invention is to provide an improved packingmaterial that can be used in a conventional stuffing box and operatedwith little or no leakage of liquid past the packing.

Still other objects and advantages of the invention will be in partobvious and will in part be apparent from the specification anddrawings.

The invention accordingly comprises the several steps and the relationof one or more of such steps with respect to each of the others, and thearticle possessing the features, properties, and the relation ofelements, which are exemplified in the following detailed disclosure,and the scope of the invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the invention, reference is had to thefollowing description taken in connection with the accompanyingdrawings, in which:

FIG. 1 is a cross-sectional view of a stuffing box for a pump or otherrotating shaft packed with packing material in accordance with anembodiment of the invention;

FIGS. 2A, 2B and 2C are cross-sectional views of packing material inaccordance with an embodiment of the invention in relative stages ofcompression;

FIG. 3 is a perspective view of an expanding inner bushing for use in apacking assembly in accordance with an embodiment of the invention; and

FIG. 4 is a cross-sectional view of the bushing of FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A flushless and waterless packing assembly in accordance with theinvention is intended to operate with little or no leakage past thepacked seal. Whereas conventional braided packing is generally operatedwith some leakage to permit heat transfer and enhance lubrication,packing in accordance with the invention has suitable heat transferproperties and a suitably low co-efficient of friction that it can beoperated with minimal or even no leakage. The improved sealing assemblyin accordance with the invention relies on a specially preparedmechanical packing or a combination of specially prepared packings thatcan be installed with an improved expanding inner bushing and a thinfollower ring.

Referring to FIG. 1, a flushless waterless packing assembly 100,constructed in accordance with a preferred embodiment of the inventionis shown installed in a stuffing box 150 around a shaft 160. To installpacking assembly 100, stuffing box 150 having a rear wall 151 is openedby loosening a set of gland nuts 71 and removing a gland follower 170from stuffing box 150. All old packing, if any, is removed and stuffingbox 150 is cleaned, if necessary. Assembly 100 is then installed overshaft 160 by first installing an optional wedge set expandable bushing300, shown in FIGS. 3 and 4. Wedge set 300 includes a female or concavehalf 301 and a male or convex half 302.

Convex half 302 is slipped over shaft 160 and a flat side 321 of convexhalf 302 is positioned against rear wall 151 of stuffing box 150.Afterwards, convex half 301 of wedge set 300 is slipped over shaft 160and positioned so that a convex face 320 of male half 302 nests in aconcave or cupped face 311 of female half 301. Wedge set 300 can also bepositioned with back wall 310 against rear wall 151. A back wall 310 ofconcave half 301 is flat and presents a surface perpendicular to shaft160. Wedge set 300 is advantageously made from a glass/TEFLON composite,with a small amount of molybdenum therein. Such material can be obtainedfrom Industrial Fluoroplastics of Salt Lake City.

A highly compressed end ring 510a of assembly 100 is then slipped overshaft 160 and pushed against back wall 310 of wedge set 300. If wedgeset 300 is not used, end ring 510a is disposed against end wall 151 ofstuffing box 150. An alternate bushing or washer, similar to gasketwasher 400, may be used instead of or in addition to wedge set 300. Oneor more moderately compressed packing rings 520 (in this case three) arethen slipped over shaft 160 and pushed against highly compressed packingring 510a. Thereafter, a second highly compressed packing ring 510b isslipped over shaft 160 and against the last moderately compressed ring520. A gasket washer 400 can then be slid against highly compressedpacking ring 510b. Gasket 400 can be made of the same material as wedgeset 300.

After packing assembly 100 is installed in stuffing box 150, glandfollower 170 is replaced and packing assembly 100 is compressed bytightening gland nuts 171. Gland nuts 171 should not be over tightenedin order to prevent damaging packing assembly 100.

The packing or combinations of packings of the invention can bemanufactured from carbon fibers having graphite particles disposedthereon. The graphite particles are in a foamed or expanded form whichis substantially inelastically compressible. The packing substantiallywill not return to its original dimensions after compression the waypolymer foams, such as styrofoam will, after styrofoam is compressed.Such packing also has substantially the same thermal coefficient ofexpansion as metal and the packing assembly of the invention willtherefore maintain its excellent seal as the shaft changes temperature.

A preferred exfoliated graphite yarn for forming packing material inaccordance with an embodiment of the invention can be manufactured byheating high purity graphite crystals to a temperature over about 1500°F. This causes a rapid accordion-like expansion of the crystals. Uponcooling, a slurry of the expanded crystals are extruded, pultruded orcoated about an approximately 96% pure graphite carrier yarn to form thefinal yarn to be braided or otherwise combined. The yarn is then sizedand stabilized, removing excess flakes of graphite in the process.

These exfoliated graphite yarns are then braided, twisted, laid up orotherwise combined to form a continuous rope of packing material. A coreof inorganic fiber combined with Inconel wire with preferably dried butunsintered polytetrafluoroethylene (PTFE) and pure carbon particlesthereon or packing braided with inorganic fiber/PTFE yarns or fibers canprovide packing material with additional resilience and greaterresistance to vapor flow. Such inorganic fiber/PTFE material isdescribed in U.S. Pat. No. 4,431,648, the contents of which areincorporated herein by reference. The rope of braided yarn can then bediagonally cut to size to fit around a shaft of selected diameter andshaped in a die to form a ring of packing material such as packing ring200 shown in FIG. 2A. The cross-sectional height of packing ring 200 isreduced by compressing ring 200 in a die to form moderately compressedpacking ring 200' of FIG. 2B. Additional compression leads to a highlycompressed packing ring 200" shown in FIG. 2C.

Unlike conventional packings, packing rings formed in accordance withthe invention should not be fully compressed prior to installation.Thus, neither ring 200' nor ring 200" should be in a fully compressedstate. Because the rings are not fully compressed prior to installation,they can mold to the precise dimensions of a shaft or packing box duringinstallation and provide an essentially water tight seal. Nevertheless,the use of additional rings in a fully compressed state is notprecluded.

In a preferred embodiment of the invention, graphite foil, such as aproduct sold under the trademark GRAPHOIL, by Union Carbide, is formedaround an inner wall 210 of ring 200 prior to compression. The graphitefoil is advantageously extended only 25% to 75% up a side wall 220,towards outer wall 230. It is preferable not to wrap the graphite foilcompletely around all walls of ring 200.

In a preferred method of manufacturing a packing ring assembly inaccordance with the invention, a set of at least three rings andpreferably 5 to 7 rings are cut to size. Graphite foil is then appliedto what will be the inner wall of the packing rings which will rubagainst the pump shaft and partially up the side walls. Thereafter, therings that are intended to be the two outside rings of a multi-ringpacking assembly are compressed in the die to about 80% of theiroriginal height. Compressions of about 75 to 85% are acceptable, as longas the material is not compressed to the fullest extent. The materialfor one of the inner rings of a multi-ring packing assembly iscompressed to only about 90% of its original height. Compressions ofabout 85 to 95% have been found to be acceptable. Preferred exfoliatedgraphite packing material can typically be compressed to a maximum ofabout 65 to 70% of its original height in either normal installation ordie forming operations. Thus, the compression amounts should be lessthan this to insure that the rings will mold to shape duringinstallation.

The inner and outer packing rings are formed with different compressionlevels for the following reasons. As the gland follower compresses thepacking assembly, the inner rings, because they are easier to compress,deform first. The higher initial compression of the outer rings aidsboth in the uniform compression of the inner rings and in preventingextrusion of the softer inner rings around the wedge set, gasket washeror stuffing box. The graphite foil on the inside wall of all ringspresents a low friction, impermeable rubbing surface to the shaft beingsealed. The packing assembly also exhibits an extremely high heattransfer coefficient. Thus, the assembly can be run dry, with suitablefriction and heat transfer properties.

Because the rings of the packing assembly of the invention haveconsiderably less initial compression than a conventional die formedpacking set, the rings, in particular the inner rings, will seatextremely well to the bore of the stuffing box and to the shaft. Thus,tightening of the gland nuts permits additional deformation of thepacking as required to completely fill any minute voids in the stuffingbox while properly conforming to the shaft. The configuration andconstruction also permits a substantially uniform unit loading to allportions of the seal. This additional tightening capability will alsopermit the packing of the invention to conform better to worn orslightly irregular shafts.

The expanding inner bushing helps prevent the extrusion of any of thepacking material into the throat or base of the stuffing box around theshaft of the device. It also helps limit the amount of suspendedparticles in the media being sealed, from entering or contacting thepacking around the shaft. This minimizes wear on the shaft and possibledamage to the inside diameter of the packing. The optional thin gasketwasher helps ensure a more uniform load as well as minimize anyextrusion of the inner or outer wall of the last packing ring, in thecase of an imperfect seal between the gland follower and shaft orstuffing box.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efficiently attained and,since certain changes may be made in carrying out the above method andin the article set forth without departing from the spirit and scope ofthe invention, it is intended that all matter contained in the abovedescription and shown in the accompanying drawings shall be interpretedas illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed and all statements of the scope of the invention which, as amatter of language, might be said to fall therebetween.

What is claimed is:
 1. An assembly for packing a seal, comprising:atleast three compressable packing rings, including a first ring, a thirdring and a second ring therebetween, the three rings formed with aplurality of carbon yarns, substantially all of which have expandedgraphite particles which are in a compressable state thereon; each ofthe at least three rings having an inner surface configured to abut ashaft, side walls extending outwardly from the inner surface and anouter surface connecting the side walls; the at least three rings beingin a compressable state of partial compression from one side wall to theother, having been compressed from a pre-compression height across theside walls to a smaller current distance across the side walls such thatthe first and third rings are in a greater state of compression than thecompression of the second ring.
 2. The assembly of claim 1, wherein thefirst, second and third packing rings include yarns formed of carbonfibers, the yarns having graphite particles in an expanded compressiblecondition thereon, the yarns with the particles thereon combined intothe form of packing material in a manner in which the particles aredispersed substantially uniformly into the thickness of said packingrings.
 3. The assembly of claim 2, wherein the yarns are formed by firstheating graphite particles to a temperature until they expand in volume,and then applying those particles to the yarns.
 4. The assembly of claim2, wherein the yarns with particles thereon are braided.
 5. The assemblyof claim 2, including graphite foil disposed on the combined yarns. 6.The assembly of claim 5, including inorganic fibers having dried butunsintered polytetrafluoroethylene thereon.
 7. The assembly of claim 1,wherein graphite foil is disposed on the combined yarns at both theinner surface and the side walls, but substantially not on the outersurface.
 8. The assembly of claim 1, wherein the distance between theside walls of the first and third rings is about 75% to 85% of thepre-compression height and the distance between the side walls of thesecond ring is 85% to 95% of the pre-compression height.
 9. The assemblyof claim 1, including graphite foil over the inner surface of the atleast three rings.
 10. The assembly of claim 8, including graphite foilover the inner surface of the at least three rings.
 11. The assembly ofclaim 1, including at least a fourth ring and a fifth ring between thefirst and third rings in a state of partial compression and the distancebetween the side walls of the fourth and fifth rings is 85% to 95% ofthe pre-compression distance.
 12. The assembly of claim 8, wherein thepartial compression of the first and third rings is about 80% and thepartial compression of the second ring is about 90%.
 13. The assembly ofclaim 1, including inorganic fibers having dried but unsinteredpolytetrafluoroethylene thereon.
 14. The assembly of claim 1, includinga two piece bushing disposed against the first ring, the bushingincluding a female half having a concave face and a male half having aconvex face, the bushing assembled so that the convex face nests in theconcave face.
 15. The assembly of claim 1, wherein partial compressionis achieved by combining the yarns with expanded particles thereon intoa rope of packing material; cutting pieces of the rope to form thepacking rings; forming the pieces of rope into ring shapes in a die andcompressing the pieces in the die to a state of partial compression lessthan the maximum compression.
 16. The assembly of claim 15, wherein thepieces for the first and third rings are compressed to about 75% to 85%of their original height and the piece for the second ring is compressedto about 85% to 95% of its original height.
 17. The assembly of claim 1,wherein graphite foil is disposed on the inner surface of the rings andon the side walls, about 25% to 75% of the way from the inner surface tothe outer surface.
 18. The assembly of claim 1, including a stuffing boxfor a pump shaft seal of a pump for liquids, wherein the packing ringsare used to pack the stuffing box and prevent any liquid from leakingpast the packing rings.
 19. The assembly of claim 18, whereinsubstantially no vapor from the liquid being pumped passes past thepacking.
 20. The assembly of claim 18, wherein the stuffing box includesa wall defining an inner bore, a shaft spaced apart from andsubstantially parallel to the inner bore and a gland follower forcompressing packing material within the stuffing box and the glandfollower is tightened to an extent to prevent liquids from seeping pastthe packing.
 21. The assembly of claim 18, wherein the yarns withexpanded particles thereon are braided together.